1. Field of the Invention
The present invention relates to an RF induction heating apparatus used for a floating-zone melting process for purifying semiconductors and manufacturing dislocation-free, single-crystal semiconductor rods, the semiconductors being of a single-element or compound, by zone-melting a rod-like semiconductor in an axial direction thereof using one or a plurality of heating coils surrounding the rod-like semiconductor.
2. Description of the Prior Art
Conventional RF induction heating apparatuses for purifying semiconductors and manufacturing dislocation-free, single-crystal semiconductor rods, the semiconductors being of a single-element or compound, using a Floating Zone (to be referred to as FZ hereinafter) process are known. A heating apparatus of this type, for example, in a single-crystal semiconductor manufacturing apparatus, is designed to manufacture a rod-like single-crystal semiconductor in such a manner that a rod-like polycrystalline material is held at the bottom of an upper shaft of the apparatus and a single-crystal seed having a small diameter is held at the top of a lower shaft thereof, one end of the polycrystalline material is melted by an RF induction heating coil surrounding the polycrystalline material and nucleated on the single-crystal seed, and then the semiconductor rod is zone-melted by relatively rotating the coil and the polycrystalline material and relatively moving them in the axial direction while a dislocation-free crystal is obtained epitaxially from the seed.
Therefore, in the apparatus of this type, in order to melt the rod-like polycrystalline material to its core within a floating zone for a short period of time, a magnetic field generated by the heating coil need be concentrated and applied to a narrow region. On the other hand, in order to homogeneously grow a single-crystal semiconductor during the zone melting without local variations in impurity, an intensity of the magnetic field to be applied to a growing interface of a single crystal with the melting zone need be controlled so as to moderately heat the growing interface, thereby moderating (delaying) heat dissipation. For this reason, conventionally, a flat single-turn heating coil is used in the RF induction heating apparatus while inner and outer diameters, a sectional shape, and the like of the heating coil are selected to optimize melting of the zone and heat control during melting the single-crystal semiconductor.
However, in the recent years, as a diameter of a single-crystal semiconductor ingot in industrial use is increased, a magnetic field need be concentrated on the melting zone while the magnetic field is applied to a narrower region to stabilize the melt of the melting zone and increase heating efficiency. In addition, in order to prevent steep changes in temperatures of the solidified portion in the vicinity of the growing interface in the axis of the single-crystal semiconductor rod, an intensity of magnetic field to be applied to the solidified portion need be controlled so that the heat can be dissipated moderately. It has been more and more difficult to make such both effects compatible with each other using a single heating coil with an increase in the diameter.
Especially, in an apparatus for manufacturing single-crystal semiconductors having a large diameter, a flat single-turn heating coil has been widely used because of its various advantages, e.g., easy zone-melting of a narrow region of a semiconductor rod, a larger current at a lower voltage applied, and hence prevention of electric discharge. However, when the thickness of such a flat single-turn heating coil and its inner diameter are decreased to improve magnetic field concentration on a floating zone, it becomes more difficult to optimize an intensity of the magnetic fields between the melting zone and the solidified portion interfacing thereto.
In general, since single-crystal semiconductors are manufactured using the FZ process in a protective gas atmosphere using argon gas or the like, a swift motion of gas flow 5 is generated near a surface of a melting zone 3 of the semiconductor rod 1, as shown in FIG. 9. In addition, since in the flat single-turn heating coil, both ends of the coil, which face to each other with a gap spaced in a predetermined distance, the gas flow 5 passes through the gap and around the outer peripheral surface of the coil, and directly collides with a peripheral edge portion 2a of polycrystalline material 2. As a result, the collided portion is locally cooled and a nonmelted portion may be left.
In the above-described FZ process, since the melting zone 3 sequentially produces a single crystal in the form of the thin layer per revolution of the semiconductor rod 4, if the nonmelted portion is generated, as described above, the nonmelted portion is sequentially left to grow an icicle-like "overhang" 6.
If the "overhang" 6 is formed, when the coil 10' is moved up to a polycrystalline heating region 4, the coil 10' collides with the "overhang" 6, and hence the coil cannot pass thereby, resulting in an interruption of the manufacture at this position.